When the good ductility, drawing, and elongation properties are to be obtained in cold-rolled strip, the strip is usually subjected to recrystallization annealing in the coiled state in a bell furnace. Such a treatment, however, is very costly for it is of long duration and thus of low productivity. Moreover, the results obtained in this way have a very large scatter.
To remedy these drawbacks, we have already suggested replacing the conventional annealing treatment with a continuous heat-treatment comprising heating the strip to a temperature higher than its recrystallization temperature, and then immersing the heated strip in an aqueous bath maintained substantially at its boiling temperature. The results thus obtained are very satisfactory, especially with strip designed to be drawn, strip having a high elastic limit, and strip with a great tensile strength and large elongation. In some cases, however, it is not easy to obtain and maintain a boiling state throughout the bath. On the contrary, it has been found that, owing to the configuration of the immersion vessel, the movement of the product and the presence of accessories (rollers, cold-water inlet conduits, etc), the aqueous bath has temperature differentials higher than 25.degree. C., which produces satisfactory but heterogeneous results.
It would be desirable to remedy the lack of thermal uniformity of the boiling aqueous bath and to increase the cooling rate. The latter characteristic is particularly advantage-in the case of mild steel because the more rapid the cooling operation the shorter the carbide-nitride precipitation phase, which results in a better ductility and thus in improved drawability.
It is known that the continuous annealing cycle, such as that for white iron, is not suitable for strip designed to be drawn. Such a continuous annealing cycle actually comprises the following four stages: heating to a temperature of about 650.degree. C., soaking at a temperature of about 700.degree. C., controlled cooling to a temperature of the order of 450.degree. C., and rapid cooling to ambient temperature. The two-stage cooling is too rapid for carbon and nitrogen to be able to precipitate in the form of carbo-nitrides. The steel thus obtained is unsuitable for deep drawing because it is too hard and of poor ductility.
For a long time it has been conventional, after a continuous annealing or annealing-galvanization operation, to subject steels to a carbon-precipitation treatment at 300.degree.-400.degree. C. in a bell furnace. Properties are thus obtained which are comparable with those obtained with a conventional method of close annealing.
Continuous methods combining the two operations have already been suggested but are too costly because the furnace needs to be very long to ensure full precipitation of carbon in steel.
The main object of the present invention is a method based on the above described considerations making it possible to obtain satisfactory uniformity of the properties over the entire width of the strip and good ductility while preserving an acceptable duration of treatment.
Another object is a substantial reduction in the duration of the treatment with respect to the conventional methods. We have found that, if rapid cooling after recrystallization is performed at two different and suitable rates, the following carbon precipitation operation is facilitated in the sense that the time necessary for carrying it out is substantially shortened.
Another important reduction in the duration of the carbon precipitation operation (overageing) may result from the division of this operation into two stages. We have also found that it is possible to precipitate carbon at a first temperature in a very short time until the amount of carbon still in the solution approximately corresponds to the equilibrium content at this first temperature. For further decreasing the carbon in solution it is then advantageous to adopt a second, lower temperature at which the equilibrium content of dissolved carbon is lower.